Loudspeaking telephones (also known as speakerphones or hands-free telephones) are ones that locate a microphone and a loudspeaker outside of a conventional telephone handset in somewhat close proximity to each other, thereby creating the opportunity for sustained oscillation to occur. This situation, known as singing, is often encountered in public address systems when signals from a loudspeaker are coupled to an associated microphone. Loudspeaking telephones generally include amplifiers in both the transmit and receive channels of a telephone set as well as a hybrid circuit that interconnects the transmit and receive channels to a telephone line. Although the hybrid circuit couples most of the transmitted signal energy to the telephone line, a portion, known as hybrid echo, finds its way back into the receive channel. In a similar manner a portion of the acoustic energy emanating from the loudspeaker is picked up by the transmitting microphone and is known as acoustic echo. Thus a loop is created that includes the transmit channel and the receive channel. They are coupled by hybrid echo at one end and by acoustic echo at the other. Oscillation occurs when the net gain around the loop exceeds unity (0 dB).
Perhaps the earliest technique used to circumvent the oscillation problem was the so-called "push-to-talk" system. In its normal state the transmit channel is disabled and the receive channel is enabled. When a user wants to talk he depresses a manual switch to enable the transmit channel and simultaneously disable the receive channel. Oscillation can never occur because the transmit and receive channels are never on at the same time. An improvement in the push-to-talk system came when the manual switch was replaced by circuitry that detected speech energy at the transmitter which thereafter enabled the transmit channel and disabled the receive channel--a technique known as voice switching. A refinement of the voice-switched system came with the inclusion of circuitry to compare the magnitude of the transmit and receive signals and enable the loudest talker to dominate. Apart from the dubious wisdom of rewarding such behavior, there is the problem of losing a syllable or two during the time that the direction of transmission is being reversed. Recognizing the desirability of full-duplex operation in a loudspeaking telephone (i.e., simultaneous conversation in two directions), other techniques are sought that reduce coupling between the loudspeaker and the microphone.
U.S. Pat. No. 4,658,425 discloses a microphone actuation control system such as used in the Shure ST 3000 Teleconferencing System. In this system, three first-order-gradient (FOG) microphones, each having a heart-shaped (cardioid) polar response pattern, share a common housing with a loudspeaker. Each of the microphones faces outward so that the direction of maximum sensitivity emanates radially from the center of the housing. The overall pattern provided by the three microphones allows full room (360.degree.) coverage, although normally only one microphone is on. Unfortunately, manufacturing variations among cardioid microphones, as well as the telephone housings that hold them, lead to the creation of side lobes in the polar response pattern. (Although other lobes may be defined, for the purposes of the present invention, all lobes other than the major lobe are designated "side" lobes.) Such unintended side lobes indicate an increased responsiveness to sounds coming from the directions toward which the side lobes point. Frequently this direction is where the loudspeaker is located, and thus the likelihood of sustained oscillation is increased.
Loudspeaking telephones also suffer from reverberation (barrel effect) in which the microphone picks up non-direct speech coming from reflections of direct speech from a wall or ceiling. Sounds emanating from the receiving loudspeaker are similarly reflected and picked up by the microphone and can create a reverberant echo from the far-end talker. U.S. Pat. No. 4,629,829 discloses a full-duplex speakerphone that uses an adaptive filter (echo canceler) to reduce acoustic coupling. Echo cancelers mitigate echos by generating an estimate of the echo and then substracting the estimate from the signal corrupted by the echo. However, echo cancelers are only useful over a limited signal range and provide the greatest benefit when the acoustic coupling between microphone and loudspeaker is minimized.
It is therefore desirable to configure a loudspeaking telephone station in a manner that provides a stable polar response pattern of its associated microphone(s) with respect to manufacturing variations.
Additionally, it is desirable to provide a loudspeaking telephone station capable of full-duplex operation in locations where reverberation and room noise exist.